When and how did the synonym of the word “rooster” become the synonym of male reproductive glands?
Wiki Answers (but beware; no ducmentation is given for their assertions)
“Cock” (penis) is not a reproductive gland. For those, the common/vulgar term is “balls”.
The OED gives us this:
So since 1618. Domestic fowl usage has been since 897
I thought there might be some correlation between the phenomenon of morning wood and the traditional role a rooster to wake people up in the morning. Apparently not, but I still like that explanation better. : )
In which culture?
For what it’s worth, the corresponding Scandinavian word is kuk. The Swedish Wiktionary claims that the two words are related, as is gök = cuckoo.
In those early cites it looks to me like cock is being used as a rather mild euphemism, which is interesting considering the Scandinavian word is so similar but goes back to norse language in its current meaning.
Hey, it’s the male bird whose only job is to mate with the females, early and often.
This is rather suggestive (from the online etymology dictionary):
Rooster -> one who struts -> pert boy -> erect penis ?
Or how about:
rooster -> stand up erect -> erec ears -> erect penis ?
I seem to remember that the double-entendre works in Balinese too but I can’t find a cite.
The English word rooster is tupp in Swedish and the expression “a frog in the throat” corresponds to “en tupp i halsen”. I know at least one person who has said after clearing her throat “Excuse me, I just got a cock in my throat”.
Or possibly earlier, as this poem, from the 14th-15th centuries, attests:
Or, in the modern vernacular:
According to this page, the poem is “attributed to the monastery of Bury St Edmonds.”
So, looking at it from a theological perspective:
“He gets me up early
My prayers to say”
and,
“every night he perches
In my lady’s chamber.”
Yes, it’s all metaphorical, no doubt. Some deep mystical meaning. 
Were there a whole lot of “lady’s chamber’s” in monasteries?
Monks and friars had a notorious reputation for lechery before the Catholic Counter-Reformation when the church authorities cleaned house.
If we can generalize to birds in general, several of Catullus’ poems use “sparrow” (“passer”, in Latin) as slang for the penis.
There’s that. Plus, of course, a monk’s as free to write non-autobiographical poetry as anyone else.
Your reference says exactly the opposite. Rooster refers to something bent. Cock in the sense of standing up derives from the position of the hammer on a firearm.
eh?
kevlaw’s cite says that the sense of “standing up” is derived from the bird’s head or tail, while “bent” is from the firearm position.
And penis ensued!
Actually in the southern US the word cock is also used for the female genitalia. Often when someone is called a cocksucker the reply is “yeah, SOUTHERN COCKSUCKER!”.
Not that I doubt your word, but the top hits on google for “southern cocksucker” are gay personal ads and this thread (!). Are you pulling my Yankee leg?